The invention relates generally to power supplies for electronic devices and more particularly to two stage power supplies for monitors.
The advances in personal computer technology has fueled an increase in demand for personal computers. A critical component of the personal computer is the computer monitor. The anticipated increase in the use of computer monitors has resulted in a stringent set of design requirements that has not been achieved in known monitor systems. For example, computer monitors should operate from line voltages of approximately 90 to 260 volts RMS, while still providing multi-mode operation. A power supply for such a monitor should operate from 110 volt or 220 volt line voltages and provide output voltages from approximately 5 to approximately 175 volts. Due to environmental emission regulations, such a power supply would also be required to achieve power factor correction of greater than 95%.
These requirements cannot be achieved cost effectively using known power supply architectures. The wide range of output voltages results in a low volts-per-turn requirement on the power supply transformer. This leads to a large number of turns in the high voltage windings, and, therefore, increased losses in the transformer windings. Linear regulators are sometimes used on low voltage outputs to address the problem of a high a volts-per-turn requirement on a transformer. This solution, however, increases cost and reduces efficiency.
The efficiency requirements are typically addressed with a soft-switching type power supply; however, such supplies do not operate well over a wide range of input voltages. The soft switcher requires an input voltage limitation so that the resonator and associated xe2x80x9csoft switchingxe2x80x9d circuitry will operate in the lowest noise, most efficient manner. Soft switchers typically have a minimum power output capability while operating in full power mode. Typical multi-mode monitor requirements include an intermediate, low power (i.e. 20W) mode of operation known as the Suspend or Standby mode. This requirement is typically below the minimum load where optimum efficiency and performance can be achieved for the typical soft switching power supply.
A power supply according to the principles of the invention meets several contradictory power supply requirements. An exemplary power supply operates from approximately 90 to 260 volts RMS, while providing high efficiency over a wide load range. A power supply according to the principles of the invention also achieves good line regulation and load regulation and provides multiple outputs from, for example, 5 to 175 volts, while achieving power factor correction of greater than 95% with low ripple. Such an exemplary power supply can achieve these requirements without use of linear regulators on the voltage outputs, although linear regulators can be used if required.
A power supply according to the principle of the inventions comprises two stages. The first stage is a power factor corrector stage. This stage provides the required line regulation, including high (such as greater than 95%) power factor correction, and provides a range of outputs from approximately 5 volts to 200 volts with good load regulation at the low voltages. The second stage is a soft switching power supply having good load regulation from approximately 40 to approximately 200 volts. As typical for soft switching power supplies, the second stage exhibits extremely high efficiency. An intermediate high voltage of the first stage (a voltage between the highest line voltage and highest output voltage) supplies the input to the second stage. The power supply thereby achieves both good line regulation and good load regulation over all outputs.
This exemplary two-stage power supply is adaptable for use in a multi-mode computer monitor or television. Since the first stage provides good power regulation at low voltages, the first stage and the second stage can be partitioned for a three-mode supply. In a suspend mode, the low voltages in the first stage can provide the xe2x80x9calways-onxe2x80x9d voltages typically required of monitors. These voltages, which range from 5 in an off mode to 15 volts in a standby mode, provide power to the microprocessors, microcontrollers, and remote control circuitry as required. The high voltages on the secondary are unloaded during the suspend mode (no picture), thereby reducing the feedback requirements. A reduction in feedback requirements also reduces the feedback components, such as optoisolators, and, therefore, the relative cost of the power supply.